Golf ball covers are typically formed from a variety of materials, including balata and ionomer resins. Balata is a natural or synthetic trans-polyisoprene rubber. Balata covered balls are favored by the more highly skilled golfers because the softness of the cover allows the player to achieve spin rates sufficient to more precisely control ball direction and distance, particularly on shorter shots.
However, balata covered balls are easily damaged, and thus lack the durability required by the average golfer. Accordingly, alternative cover compositions have been developed in an attempt to provide balls with spin rates and a feel approaching those of balata covered balls, while also providing a golf ball with a higher durability and overall distance.
Ionomer resins have, to a large extent, replaced balata as a cover stock material. Chemically, ionomer resins are a copolymer of an olefin and an alpha, beta ethylenically unsaturated carboxylic acid having the carboxylic acid groups neutralized to various extent by a metal ion. Commercially available ionomer resins include, for example, copolymers of ethylene and methacrylic or acrylic acid neutralized with metal salts. These are sold by E. I. DuPont de Nemours and Co. under the trademark SURLYN® and by the Exxon Corporation under the trademark ESCOR® and the trademark IOTEK®. These ionomer resins are distinguished by type of metal ion, amount of acid, and degree of neutralization. While SURLYN® covered golf balls possess virtually cut-proof covers, they have inferior spin and feel properties as compared to balata covered balls.
Polyurethanes have also been recognized as useful materials for golf ball covers since as early as about 1960. For example, U.S. Pat. No. 3,147,324 is directed to a method of making a golf ball having a polyurethane cover. The curing agents disclosed are diamines, polyols or air moisture. The disclosed polyurethane covered golf balls are durable, while at the same time maintaining the “feel” of a balata ball.
Various companies have investigated the usefulness of polyurethane as a golf ball cover material. For example, U.S. Pat. No. 4,123,061 discloses a golf ball made from a polyurethane prepolymer of polyether and a curing agent, such as a trifunctional polyol, a tetrafunctional polyol or a diamine. U.S. Pat. No. 5,334,673 discloses the use of thermoset and thermoplastic polyurethanes for forming golf ball covers, and in particular, thermoset polyurethane covered golf balls made from a composition of polyurethane prepolymer and a slow-reacting amine curing agent and/or a difunctional glycol.
Unlike SURLYN® covered golf balls, polyurethane golf ball covers can be formulated to possess soft cover characteristics. However, golf ball covers made from polyurethane have not, to date, fully matched SURLYN® golf balls with respect to resilience or the rebound of the golf ball cover, which is a function of the initial velocity of a golf ball after impact with a golf club.
Polyureas have also been proposed as cover materials for golf balls. For instance, U.S. Pat. No. 5,484,870 discloses a polyurea composition comprising the reaction product of an organic diisocyanate and an organic amine, each having at least two functional groups. Like polyurethanes, polyureas are not completely comparable to SURLYN® golf balls with respect to resilience or the rebound or damping behavior of the golf ball cover.
To improve the playing characteristics of polyurethane covered golf balls, the use of cationic polyurethane ionomers has been proposed in U.S. Pat. No. 5,691,066. Although, the inclusion of cationic polyurethane ionomers in a golf ball cover can further increase the resilience of the ball, there seems to be a limitation in this approach due to the limited choice of suitable alkylating agents used in the processing of such cationic polyurethanes.
Therefore, there remains a continuing need for balls having soft covers that provide improved resilience, increased cut, scratch and abrasion resistance, and enhanced adherence without adversely affecting overall performance characteristics of the golf balls.